std::string is designed expressly to allow you to treat strings as simple pass-by-value types, like int. It's efficient to return by value rather than reference and it avoids core dumps if the real string hidden away in f gets deleted before the reference. In particular it allows f() to compute once-off values and forget about them, e.g.:

With the "&" style return this would be an immediate disaster as the returned reference is invalid before the caller even gets it! NB. The last example contains another error! See CppTips/BewareOfStringPromotion.